This invention relates to hoists for raising and lowering the dump beds of trucks. More particularly, this invention relates to improved dual acting truck hoists provided with unique safety locking mechanisms.
Extendible and retractable hoists placed between the frame of a vehicle and its dump bed (body) for raising and lowering the dump bed about a pivoting hinge are well known in the art and come in many different types and designs, whether they be rear or side dump vehicles. The term xe2x80x9ctruckxe2x80x9d is used herein to describe all types of vehicles which employ such hoists, including light, medium and heavy duty dump trucks and trailers.
In a typical hydraulic truck hoist, one or more hydraulic cylinders lie lengthwise between the frame rails of the vehicle and underneath the truck bed. As a hoist""s cylinder extends, one end of the hoist pushes against the underside of the dump bed while the other end of the hoist pushes against the frame of the truck to which it is pivotally attached. This causes the bed to rotate about its rear hinge pivot whereby the dump bed is raised. Retraction of the cylinder causes the bed to be lowered.
In many designs, the cylinder is nearly parallel to the dump bed when in its lowered position, and thus has very little initial leverage at the beginning of the lifting process. During this initial lift period most of the force of the cylinder is directed towards the rear hinge rather than in the more efficient raising direction, which changes as the body is raised. Due to this inefficiency, larger cylinders than would otherwise be used, are required in practice to be used, to safely and effectively perform the initial lifting of the dump bed, at least until the cylinder(s) and bed reach a height at which the angle of the extending cylinder is force efficient and thus a smaller designed cylinder would then be adequate. As is rather apparent, the size of the cylinder can not be changed during lifting and thus a large enough cylinder(s) to accommodate the initial inefficient lifting process must be used.
Many types of hydraulic hoists have been developed to address this initial lift problem. In one example, telescopic cylinders are mounted farther forward on the truck frame. This provides the cylinder with greater initial leverage. The cylinder in such devices usually must have a rather long stroke to achieve a sufficient dump angle (i.e. height of lift of the dump bed), which is a known disadvantage in the art.
Other attempts to improve these hoists have included the use of a simple lever arm to provide the lifting motion in an attempt to obtain a more efficient initial lift angle. Examples include those hoists disclosed in U.S. Pat. No. 4,052,105. Other designs include those disclosed in U.S. Pat. Nos. 2,509,911; 2,603,518; and 4,762,370. These patents disclose hoist designs in which a lever arm aids the cylinder during initial lift of the truck bed, and thereafter the cylinder completes the lifting process. Such systems are called xe2x80x9cdual actingxe2x80x9d systems at times, and the lever arms may be referred to as xe2x80x9cboosterxe2x80x9d arms. This invention adopts in its preferred embodiments this xe2x80x9cdual actingxe2x80x9d concept and, as described below, improves upon former designs in this regard.
Another problem faced in the dump truck art is the potential for operator abuse of the system if a portion of the load being carried in the dump bed (e.g. wet dirt) xe2x80x9changs upxe2x80x9d in the bed during the dumping operation and thus is not dumped as intended. In such situations, it is not unusual for the truck operator, with the bed fully raised, to put the transmission in gear, accelerate the engine and quickly xe2x80x9cpopxe2x80x9d the clutch, causing the truck to lurch, usually forward. As the truck lurches, the operator then quickly steps forcefully on the brake pedal. As the truck lurches, various parts of the hoist, as well as the dump bed, are pulled away from their established positions with respect to each other. Then, when the brakes are suddenly applied, the parts under force added to by the heavy dump bed, rebound back against those elements in the system which established the original position. The maneuver itself is designed to use the jarring effect of the rebound to dislodge the hungup portion of the load in the bed, and it is not unusual for the maneuver to be repeated several times to insure the intended result. Significant damage to various truck hoist parts, the hydraulic system, the truck, and the dump bed itself can result from this practice. A hoist which prevents or discourages this practice, or at least mitigates the potential for damage if the maneuver is conducted, is clearly needed in this art, and it is an object of this invention in certain preferred embodiments to provide such a hoist design.
In one reported design, found in U.S. Pat. No. 4,762,370, a dual acting hoist mechanism is disclosed which appears to seek to address a tipping problem in dump trucks. In this design the hoist system employs a hook, tie rod and slide block arrangement which latches to a plate member when the dump bed reaches its ultimate height, thereby purportedly achieving a measure of stability against tipping. In such a design, in addition to having to employ a friction sensitive sliding block and rod mechanism (adding complexity to the system), there appears to be required a special straight line alignment of pivots in order for the system to operate as intended. There is no recognition, moreover, that the hook arrangement used is adequate in size and design to overcome the aforesaid problem caused by the truck operator""s maneuver to free the bed of hungup load material.
In view of the above, it is apparent that there exists a need in the art for an improved latching mechanism which overcomes, mitigates or solves the above problems in the art. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.
Generally speaking this invention fulfills the above needs in the art by providing:
a hoist for raising and lowering a truck bed to and from a truck frame, the hoist comprising:
an extendible and retractable cylinder having a first end and a second end;
means for attaching the second end of the cylinder to the truck bed;
a lever mechanism comprising a base member, an elongated arm having a first end and a second end, a first pivot connecting the first end of the cylinder to the first end of the elongated arm, a second pivot connecting the base member to the elongated arm at a location intermediate the first and second ends of the elongated arm, means located proximal the second end of the elongated arm for contacting the truck bed, a stop mechanism for limiting the extent of rotation of the elongated arm and the first pivot about the second pivot and so located that when the stop mechanism is engaged to prevent further rotation of the elongated arm, further extension of the cylinder disengages the means proximal the second end from the truck bed; and
a latch mechanism comprising a latching member and a latch retaining member, the latching member having a first end connected to the first end of the cylinder and a second end for latching engagement with the latch retaining member,
the latch retaining member and the stop mechanism being so located that when the latch member is in latching engagement with the latch retaining member and the stop mechanism is engaged any further substantial rotation in either direction of the first pivot with respect to the second pivot is prevented unless the cylinder is retracted.
By employing the above hoist, it can be seen that the intended movement of the truck bed during the above-described maneuver of xe2x80x9cpoppingxe2x80x9d the truck""s clutch and stepping on the brakes as the truck lurches, to thereby cause the bed to rebound against other parts of the system (hopefully to thus jar loose cargo stuck in the truck bed) is simply and effectively prevented or at least minimized by prohibiting any significant separating movement of the extended cylinder system parts. In short, the bed and hoist system are caused to follow the movement of the truck if the maneuver is attempted and no substantial rebound occurs.
In a preferred embodiment of this invention, the cylinder employed is a telescopic cylinder comprised of a base cylinder and at least one tubular smaller cylinder or rod extending from the base cylinder, wherein the first end of the smaller rod is connected to the first pivot and its opposite second end is connected to the next largest cylinder or base cylinder such that the base cylinder is connected to the truck bed. It is understood, of course, that while only one cylinder is hereinafter shown and discussed in the embodiments as illustrated, two or more cylinders may be used in a known and conventional construction to act in parallel. Moreover, while both a single stage cylinder and telescopic cylinder are illustrated, either cylinder type, where appropriate, is contemplated for use therein.
In further preferred embodiments, the connection of the tubular cylinder to the first pivot comprises the conventional crosshead member of a hydraulic cylinder.
In still further preferred embodiments, the latch member comprises a hook whose base end is rigidly attached to the crosshead member of the cylinder, and the crosshead member is part of a strong, welded assembly which includes the tubular rod, a retaining plate, a bushing crosshead and the hook.
In a still further preferred embodiment of this invention, the first pivot is offset from the second pivot in a manner described below thereby to achieve certain unique results.
This invention will now be described with respect to certain embodiments thereof as illustrated in the following drawings, wherein: